Linux Log Files

Managing Log files on a Linux System

Linux's Log Files

All Linux systems generate systems logs that can be inspected to find information about your running system. These log files can contain a wealth of information from simple information messages to critical system issues. Most of the logging files that are created are in plain text. This means that it is very easy to view these using standard commands such as "more", "less", "cat", "head", "tail", "pg" or by using your preferred text editor such as "vi or vim".

By convention, most of the log files that are created are found under the directory "/var/log/". This is a standard area where system messages and logged/recorded. Depending on which Linux distribution you are using you will probably have a "message" file or a "syslog" file that contains recent activity. Logfiles are generally created by either a "syslogd" or "rsyslogd" logging demon. These demons are highly configurable and can filter messages into specified files. As well as handling local events, it is possible to log messages to remote servers dedicated to receiving these type of messages. It is quite common within larger organisations to have a dedicated syslog server. Some basic configuration options will be covered later. It is also common practice to have some form of log rotation process.

Below is a list of some of the more common log files that you will find. Some of these are distribution specific:

Log File Description
/var/log/messages Global system messages are logged here. (default logging area on some systems)
/var/log/syslog Global System messages are logged here. (default logging area on some systems)
/var/log/auth.log System Authorisation information, including user login information
/var/log/kern.log Kernel messages are logged here
/var/log/mail.log Contains logging information from your mail server
/var/log/boot.log System boot messages are logged here
/var/log/cups.log Printer related messages logged here
/var/log/wtmp Contains information relating to users logged onto your system
/var/log/samba Samba log files for smbd and nmbd. If configured can contain specific log files for users.
/var/log/dpkg.log Contains information from installations that use dpkg to install or remove a package
/var/log/zypper.log Contains messages from zypper package manager tool
/var/log/apt Contains information from package updates that use APT
/var/log/dmesg Contains Kernel ring buffer messages

Although the above is not an exhaustive view of all the files that can be found within the "/var/log" area. It does give you a rough idea of what is logged. It is important to remember that many third part products (software) will also write to this area. Often a sub-directory is created with various log information held within. Samba is a good example of this.

As mentioned earlier it is the "syslogd" or "rsyslogd" daemon that handles the majority of logging on your systems.

rsyslogd - logging utility

Rsyslogd is a reliable extended version of the syslogd service. Linux uses rsyslogd as its mechanism to record log files either in a central area or split into separate directories for clarity. It is also possible to send information to a dedicated logging server. Multiple processes may write to the same area without causing file locking. Simple commands can be used directly from scripts to write to this area.

Configuration Files

How rsyslogd behaves on your system is down to its configuration. This file is generally located in "/etc/rsyslog.conf". This file contains text which describes what should happen to messages when they are logged. It is here that you can specify specific directories for specific message types. Default logging rules are generally located under "/etc/rsyslog.d/"

Example of rsyslogd.conf under Ubuntu


#  Default logging rules can be found in /etc/rsyslog.d/50-default.conf

#### MODULES ####

$ModLoad imuxsock # provides support for local system logging
$ModLoad imklog   # provides kernel logging support (previously done by rklogd)
#$ModLoad immark  # provides --MARK-- message capability

# provides UDP syslog reception
#$ModLoad imudp
#$UDPServerRun 514

# provides TCP syslog reception
#$ModLoad imtcp
#$InputTCPServerRun 514

# Use traditional timestamp format.
# To enable high precision timestamps, comment out the following line.
$ActionFileDefaultTemplate RSYSLOG_TraditionalFileFormat

# Filter duplicated messages
$RepeatedMsgReduction on

# Set the default permissions for all log files.
$FileOwner syslog
$FileGroup adm
$FileCreateMode 0640
$DirCreateMode 0755
$Umask 0022
$PrivDropToUser syslog
$PrivDropToGroup syslog
# Where to place spool files
$WorkDirectory /var/spool/rsyslog

# Include all config files in /etc/rsyslog.d/
$IncludeConfig /etc/rsyslog.d/*.conf

Hashes "#" are used to denote a comment or for commenting out a function that is not required.
Notice the last line $IncludeConfig /etc/rsyslog.d/*.conf. This is where we can specify custom rules/mappings.

#  Default rules for rsyslog.
#			For more information see rsyslog.conf(5) and /etc/rsyslog.conf

# First some standard log files.  Log by facility.
auth,authpriv.*			/var/log/auth.log
*.*;auth,authpriv.none		-/var/log/syslog
#cron.*				/var/log/cron.log
#daemon.*			-/var/log/daemon.log
kern.*				-/var/log/kern.log
#lpr.*				-/var/log/lpr.log
mail.*				-/var/log/mail.log
#user.*				-/var/log/user.log

# Logging for the mail system.  Split it up so that
# it is easy to write scripts to parse these files.
#			-/var/log/
#mail.warn			-/var/log/mail.warn
mail.err			/var/log/mail.err

# Logging for INN news system.
news.crit			/var/log/news/news.crit
news.err			/var/log/news/news.err
news.notice			-/var/log/news/news.notice

# Some "catch-all" log files.
#	auth,authpriv.none;\
#	news.none;mail.none	-/var/log/debug
#	auth,authpriv.none;\
#	cron,daemon.none;\
#	mail,news.none		-/var/log/messages

# Emergencies are sent to everybody logged in.
*.emerg                                :omusrmsg:*

# I like to have messages displayed on the console, but only on a virtual
# console I usually leave idle.
#	news.=crit;news.=err;news.=notice;\
#	*.=debug;*.=info;\
#	*.=notice;*.=warn	/dev/tty8

# The named pipe /dev/xconsole is for the `xconsole' utility.  To use it,
# you must invoke `xconsole' with the `-file' option:
#    $ xconsole -file /dev/xconsole [...]
# NOTE: adjust the list below, or you'll go crazy if you have a reasonably
#      busy site..
	*.=notice;*.=warn	|/dev/xconsole

The default logging area is called "syslog", see below exert

*.*;auth,authpriv.none		-/var/log/syslog

What are Facilities and Levels?

Whenever the rsyslogd daemon receives a logging message, it acts based on the message type (Facility) and a Level (Priority). These mappings can be seen in your "/etc/syslog.conf" file or your included "/etc/syslog.d/*.conf" files.

Each entry within the configuration file can specify one or more facility/level selectors followed by an action. A selector consists of a facility or facilities followed by a single action. Action is normally the name of the directory and file that is to receive the messages into.

facility.level action

Example: mail.* -/var/log/mail - Here, "mail" is the facility, level is set to "*" and action is "/var/log/mail"


A facility represents the creator of the message, these normally consist of:

auth, authpriv, cron, daemon, kern, lpr, mail, mark, news, syslog, user, local0 - local7, "*" signifies any facility

These facilities give us the ability to control where messages from certain sources are sent to. The facilities local0 - local7 are for use by your own scripts.

Level (Priority)

The level specifies the severity threshold. These can be: (lowest priority first)

debug, info, notice, warning, err, crit, alert, emerg.

On older systems you may see "warn, error or panic". A level of none will disable the associated facility. These priorities control the amount of detail that is sent to each logfile. A single period "." separates the facility from the level. Together these are known as the message selector. An asterisk "*" may be used to specify all facilities or levels. Similar to facilities, wildcards "*" can be used along with "none". Only one level or wildcard may be specified with each selector. The following modifiers may be used "=" and "!"

If you specify only one level within a selector without any modifiers, you are actually specifying that level plus all other priorities. For example the selector user.notice is actually saying all user related messages having a level of notice or higher will be sent to the specified action area. If you require only a level of "notice", then you will have to use the "=" modifier:

user.=notice - Now means any user related messages with a level priority of "notice" only will be sent to the relevant logging area.

If you use the "!" modifier, this will negate your level priority. So if we specified user.!notice is the equivalent of all user related messages with a level priority of "notice" or higher. You can also specify user.!=notice which specify all user related messages except for ones with the level priority of "notice".


The action section is the destination for the messages. The action can be a filename such as "/var/log/syslog" or a hostname or IP address prefixed with the "@" sign. The latter option is popular in large organisations and enterprises. Quite often security related messages may be sent to a central logging server for further scrutiny.

rsyslog.conf structures

As rsyslogd is an enhanced version of the syslogd it can handle the older legacy style constructs known as sysklogd. It also handles legacy versions of rsyslog. However, the true power of rsyslog comes into play when you use what is known as "RainerScript". This is the new style format for rsyslog which can handle complex cases with ease. In the example below you can see old format entries along with newer entries that use "if - then" constructs for a more precise handling.

Example section of "/etc/rsyslog.conf taken from openSUSE

# NetworkManager into separate file and stop their further processing
if      ($programname == 'NetworkManager') or \
        ($programname startswith 'nm-') \
then    -/var/log/NetworkManager
&       ~

# email-messages
mail.*                                  -/var/log/mail                               -/var/log/
mail.warning                            -/var/log/mail.warn
mail.err                                 /var/log/mail.err

# news-messages
news.crit                               -/var/log/news/news.crit
news.err                                -/var/log/news/news.err
news.notice                             -/var/log/news/news.notice

Message Testing with the logger command

logger is a shell command interface into the syslog module. Logger allows you to make entries directly into the system log. This is very useful when incorporated into a script or when you want to test your message selector and mappings.

In its simplest form we can issue logger "I am a test". This message would then go to our default area (probably /var/log/syslog or /var/log/messages) depending on how you have configured your rules. You can also specify a priority using the "-p or --priority" option. Examples of logger inaction:

john@john-desktop:/var/log$ logger "I am a Test of logger"

Mar 22 22:39:51 john-desktop kernel: [ 9588.319477] dev_remove_pack: edad0884 not found.
Mar 22 22:45:01 john-desktop CRON[4087]: (root) CMD (command -v debian-sa1 > /dev/null && debian-sa1 1 1)
Mar 22 22:47:31 john-desktop john: I am a Test of logger

Basic Logger Usage

 logger [options] [message]

 -d, --udp             use UDP (TCP is default)
 -i, --id              log the process ID too
 -f, --file <file>     log the contents of this file
 -h, --help            display this help text and exit
 -n, --server <name>   write to this remote syslog server
 -P, --port <number>   use this UDP port
 -p, --priority <prio> mark given message with this priority
 -s, --stderr          output message to standard error as well
 -t, --tag <tag>       mark every line with this tag
 -u, --socket <socket> write to this Unix socket
 -V, --version         output version information and exit

Getting help with Rsyslog

The above is intended as an overview to the processes that takes place for logging of messages to occur. For further information you can issue "man rsyslogd" from your console for an overview of the many options. For further reading you can head to the main "rsyslog" website:


"dmesg" is a special command that stands for display message. dmesg will display the message buffer of the kernel. dmesg is very useful if you want to view the messages that flew past your screen quickly during the boot process. Another useful trick is to redirect the output from the dmesg command to a temporary file: dmesg > /tmp/temp.txt.

dmesg is also useful if you are having issues with an I/O device or a "USB" device. dmesg can be used in combination with the grep command to find exactly what you are looking for quickly: dmesg | grep -i usb

dmesg basic Overview

 dmesg [options]

 -C, --clear                 clear the kernel ring buffer
 -c, --read-clear            read and clear all messages
 -D, --console-off           disable printing messages to console
 -d, --show-delta            show time delta between printed messages
 -E, --console-on            enable printing messages to console
 -f, --facility <list>       restrict output to defined facilities
 -h, --help                  display this help and exit
 -k, --kernel                display kernel messages
 -l, --level <list>          restrict output to defined levels
 -n, --console-level <level> set level of messages printed to console
 -r, --raw                   print the raw message buffer
 -s, --buffer-size <size>    buffer size to query the kernel ring buffer
 -T, --ctime                 show human readable timestamp (could be 
                             inaccurate if you have used SUSPEND/RESUME)
 -t, --notime                don't print messages timestamp
 -u, --userspace             display userspace messages
 -V, --version               output version information and exit
 -x, --decode                decode facility and level to readable string

Supported log facilities:
    kern - kernel messages
    user - random user-level messages
    mail - mail system
  daemon - system daemons
    auth - security/authorisation messages
  syslog - messages generated internally by syslogd
     lpr - line printer subsystem
    news - network news subsystem

Supported log levels (priorities):
   emerg - system is unusable
   alert - action must be taken immediately
    crit - critical conditions
     err - error conditions
    warn - warning conditions
  notice - normal but significant condition
    info - informational
   debug - debug-level messages